(1) Field of the Invention
The present invention relates to a noise reducing apparatus for reducing a noise component included in a video signal.
(2) Description of the Prior Art
Noise reducing apparatus for reducing luminance noise or color noise are widely employed in home video cassette recorders used for replaying video. Noise reducing apparatus using a line-recursive process are generally used, however, these noise reducing apparatus have a problem. Specifically, a downward color bleeding phenomenon occurs at an area in which picture brightness varys vertically in the picture. This area is the vertical boundary area.
In order to solve the above problem, for example, Japanese Laid-Open No. 2-(1990)-231890, incorporated herein by reference, discloses a method which is employed to detect a color signal corresponding to the vertical boundary area and to control that vertical boundary area using a coefficient value k in a coefficient multiplier.
A noise reducing apparatus is accordance with the prior art is explained below with reference to FIG. 1 to FIG. 3. FIG. 1 is a block diagram of a noise reducing apparatus in accordance with the prior art. A color signal including noise is provided to a color signal input terminal 200 and is supplied from a color signal output terminal 201 via an adder 205. The output signal is provided to a 1H delay device 202 and is delayed by 1H, where H is equal to a horizontal scanning period. The 1H delayed color signal is subtracted from the input color signal at a first subtracter 203 to produce a vertical boundary color signal including noise.
The vertical boundary color signal including noise is obtained at a second subtracter 206 and then provided to a coefficient controller 207. At the coefficient controller 207, the noise component included in the input signal is clipped to produce a control signal corresponding to a level of the vertical boundary signal. The control signal is then provided to a coefficient multiplier 204. A circuit diagram, for example, of the variable coefficient multiplier 204 is shown in FIG. 2. The output of the coefficient controller 207 is provided to a first control signal input terminal 208 (shown in FIG. 2) as a control signal. The input signal is provided to a second control signal input terminal 209 which is gain controlled according to the control signal provided to the first control signal input terminal 208 to produce an output signal at output terminal 210. FIG. 3 shows the coefficient control characteristic, that is the gain control characteristic, of the coefficient controller 207 shown in FIG. 1.
However, in the prior art configuration discussed above, the gain control characteristic is constant against the signal level of the vertical boundary segment. As a result, downward color bleeding may easily occur for a vertical boundary signal with a small amplitude.